The present invention relates to a semiconductor structure and in particular to a copper-based bonding structure with a surface protection layer.
Conventional semiconductor devices typically comprise a semiconductor substrate, normally of doped monocrystalline silicon, and a plurality of sequentially formed inter-layer dielectrics and interconnected metallization layers defining conductive patterns. An integrated circuit is formed containing a plurality of conductive patterns comprising conductive lines separated by interwiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines and logic interconnect lines. Typically, the conductive patterns on different metallization layers are electrically connected by a conductive plug filling with a via opening, while a conductive plug filling a contact opening establishes electrical contact with an active region on a semiconductor substrate, such as a source/drain region. Conductive lines are formed in trenches which typically extend substantially horizontal with respect to the semiconductor substrate. Semiconductor chips comprising five or more levels of metallization are becoming more prevalent as device geometries shrink to submicron levels.
Copper (Cu) and copper alloys have received considerable attention as a replacement material for aluminum (Al) in ultra large scale interconnection metalizations. Copper is relatively inexpensive, easy to process, has lower resistance than aluminum, and has improved electrical properties over tungsten (W), making copper a desirable metal for use as a conductive plug as well as conductive wiring.
In the formation of copper interconnects using a damascene metallization process, copper is exposed in a bonding pad area. The bonding pad area is located on the top surface of the integrated circuit structure formed on the semiconductor substrate. The bonding pad area is the region where wires make contact with bonding pads to form electrical connection with the Cu interconnects. In this case, where the copper interconnects are exposed in the bonding pad area, the copper can be designed to act as an interconnect as well as a bonding pad.
Conventional techniques for wire bonding, however, are not compatible with bonding pads comprising Cu. Existing bonding techniques such as wedge bonding and ultrasonic bonding require thermal agitation, that is, rubbing the wire against the bonding pad to form a bond therebetween. The existing technology works for the bonding of either gold wires or aluminum wires to aluminum pads. However, such technologies do not work for the bonding of gold wires or aluminum wires to copper pads, since copper is easily oxidized, forming copper oxide which is an insulator.
In U.S. Pat. No. 5,785,236 issued to Cheung et. al., methodology is disclosed for electrically connecting wires to a Cu interconnect by forming an intermediate Al pad on the Cu interconnect. In U.S. Pat. No. 6,239,494 issued to Besser et. al. another methodology is disclosed for electrically connecting wires to a Cu interconnect by forming an Al pad and an intermediate diffusion barrier on the Cu interconnect. In both described patents, uses the Al pad is used, however, disadvantageously, as it increases the resistance of the interconnection system.
A need therefore exists to form a reliable copper pad so that conventional wire bonding technology can be employed.